This invention relates generally to a cooling passage for an airfoil. More particularly, this invention relates to a core assembly for the formation of cooling passages for an airfoil.
A gas turbine engine typically includes a plurality of turbine blades that transform energy from a mainstream of combustion gasses into mechanical energy that rotates and drives a compressor. Each of the turbine blades includes an airfoil section that generates the rotational energy desired to drive the compressor from the flow of main combustion gasses.
The turbine blade assembly is exposed to the hot combustion gasses exhausted from the combustor of the gas turbine engine. The temperature of the combustion gasses exhausted through and over the turbine blade assemblies can decrease the useful life of a turbine blade assembly. It is for this reason that each turbine blade is provided with a plurality of cooling air passages. Cooling air is fed through each of the turbine blades and exhausted out film holes on the surface of the turbine blade. The position of the film holes on the turbine blade creates a layer of cooling air over the surfaces of the turbine blade. The cooling air insulates the turbine blade from the hot combustion gasses. By insulating the turbine blade from exposure to the hot combustion gasses the turbine blade reliability and useful life is greatly extended.
Typically, the cooling passages within a turbine blade are formed by a ceramic core that is provided with and surrounded with molten material that is used to form the turbine blade. Once the molten material utilized to form the turbine blade is solidified the core material is removed. Removing the core material leaves the desired cooling air passages along with the desired configuration of film cooling holes.
As appreciated, each turbine blade assembly represents a dead end or an end of a cooling airflow path. This is so because cooling air flowing from an inner side or platform of the turbine blade flow radially outward to a tip of the turbine blade. The tip of the turbine blade is closed off forming the end of the cooling air passage. Accordingly, the only exit for cooling air through the turbine blade is through the plurality of the film cooling holes disposed about and on the surface of the turbine blade. The configuration and quantity of the film holes for cooling the turbine blade is determined to produce a desired flow rate of cooling air.
The shape of the turbine blade varies throughout the cross section from a leading edge of the turbine blade to a trailing edge. The leading edge is most often much thicker than the trailing edge. However, the cooling needs in the trailing edge are often greater than those in the leading edge and therefore require cooling passages arranged within a close proximity to the trailing edge. As appreciated, cooling passages within the thinner edge section are much smaller. The smaller cooling passages require smaller core assemblies to form those cooling passages. As the size of the core assemblies are reduced the susceptibility to damage during the molding operation increases. The smaller core assemblies required the desired cooling passage in the thinner sections of the turbine blade and are more susceptible to damage during manufacturing.
Accordingly, it is desirable to develop a core assembly that is robust enough to provide for reliable manufacturing process results while still providing for the formation of the smaller cooling air passages in the thinner sections of the turbine blade assembly.
Another concern in the design and configuration of cooling air passages is the direction of cooling air on an inner side of the cooling passage. The cooling passage typically receives air from a main core section. The main core section of the turbine blade is in turn in communication with a cooling air source. The cooling air passage therefore includes an inner surface that is adjacent the main core and an outer surface that is adjacent an exterior surface of the turbine blade. Impingement holes within the cooling air passages communicate air from the main core into the cooling air passage and against the outer surface.
Accordingly, it is desirable to develop a core assembly to form a cooling air passage within a turbine blade assembly that is both reliable during manufacturing processes and that provides the desirable cooling air flow properties to maximize to heat transfer capabilities applications.